The present invention relates to an optical fiber collimator and an optical fiber collimator array in which a gradient index rod lens is used.
An optical collimator device includes first and second optical fiber collimators. The first optical fiber collimator converts a light emitted from an optical fiber into a parallel light. The second optical fiber collimator converges the parallel light into a collimator lens and couples the light to an optical fiber. The first optical fiber collimator will hereinafter be referred to as an optical fiber collimator on an incidence side, and the second optical fiber collimator will be referred to as an optical fiber collimator on a reception side. In general, the collimators on the incidence and reception sides have substantially the same structure.
Optical function devices such as an optical filter, optical isolator, and optical switch are inserted between two collimator lenses of the optical collimator device. The optical collimator device exerts a predetermined action onto the light propagated through the optical fiber on the incidence side by the optical function device, and subsequently couples the light to the optical fiber of the optical fiber collimator on the reception side.
FIG. 1 is a schematic constitution diagram of an optical fiber collimator 20 for use in a conventional optical collimator device. The optical fiber collimator 20 includes a gradient index rod lens 21, single mode optical fiber 22, capillary 23 which holds the optical fiber 22, and glass tube 24 which holds the capillary 23 and rod lens 21. Anti-reflection films 25, 26 are formed on both end faces of the rod lens 21, and an anti-reflection film 27 is also formed on the end face (single mode fiber (SMF) end face) of the optical fiber 22. The anti-reflection films 25 to 27 are usually dielectric multilayered films.
The anti-reflection films 25 to 27 prevent the incident light from the optical fiber 22 from being reflected and returned to a light source side. Moreover, a loss of the incident light caused by the reflection is suppressed. In the optical fiber collimator 20, the end faces of the optical fiber 22 and rod lens 21 are obliquely polished. The oblique polishing can prevent the reflected light from the end faces of the optical fiber 22 and rod lens 21 from being incident upon the optical fiber 22 again and returning to the light source side. If the rod lens 21 is coupled to the optical fiber 22 without taking any anti-reflection countermeasure, the following disadvantage occurs.
FIG. 2 shows only a rod lens 21A and optical fiber 22A of an optical fiber collimator 20A which does not have any anti-reflection film. In the optical fiber collimator 20A, both end faces 21a, 21b of the rod lens 21A are flat surfaces vertical to an optical axis, and the anti-reflection films are not formed on the end faces 21a, 21b. The SMF end face of the optical fiber 22A is a flat surface vertical to a core center axis, and the anti-reflection film is not formed also on the SMF end face.
In the optical fiber collimator 20A not subjected to the anti-reflection countermeasure, the reflection by each of the SMF end face and the end faces 21a, 21b of the rod lens 21A occurs substantially by 5%, and a loss of about 14% is generated as a whole. As a result, a coupling efficiency is xe2x88x920.6 dB or less, and standard required characteristics such as a characteristic of xe2x88x920.2 dB or more cannot be obtained.
The reflected light on the SMF end face and the reflected lights on the end faces 21a, 21b of the rod lens 21A are directly incident as return lights upon the optical fiber 22A again, and return to the light source side. Therefore, the anti-reflection countermeasure is required as in the optical fiber collimator 20 shown in FIG. 1. As shown in FIGS. 1 and 3, anti-reflection films 25, 26, 27 each having a reflectance of 0.2% or less are disposed on the both end faces of the rode lens 21 and the SMF end face of the optical fiber 22. The anti-reflection films 25, 26, 27 reduce the whole reflection loss to about 0.6%. Furthermore, the SMF end face and one end face of the rod lens 21 are obliquely polished. By the anti-reflection countermeasure of the oblique polishing, the reflected return lights from the SMF end face and the both end faces of the rod lens 21 are remarkably reduced.
However, the conventional optical fiber collimator 20 shown in FIG. 1 has the following problems.
(A) It is necessary to form the anti-reflection film 27 on the SMF end face of the optical fiber 22, but it is not easy to form the anti-reflection film 27 on the SMF end face which has a long dimension and small diameter.
(B) The oblique polishing of at least one end face of the rod lens 21 and the SMF end face of the optical fiber 22 requires long time, causes the reduction of a yield, and deteriorates productivity.
(C) Since the capillary 23 and glass tube 24 are required, the number of components and the number of assembly processes increase, and cost increases.
(D) To perform an optical adjustment so that the optical axis of the rod lens 21 agrees with a core center of the optical fiber 22, it is necessary to consider the deflection of the light by the light reflection in the SMF end face of the optical fiber 22 and one end face of the rod lens 21 as the oblique surfaces. Therefore, the optical adjustment becomes complicated.
An object of the present invention is to provide an optical fiber collimator and optical fiber collimator array in which manufacturing and assembling processes are simplified, productivity is enhanced, cost is reduced, and optical adjustment is facilitated.
According to one aspect of the present invention, there is provided an optical fiber collimator including a gradient index rod lens and an optical fiber optically connected to the rod lens. The optical fiber collimator includes an anti-reflection film formed on one end face of the rod lens. The anti-reflection film has a refractive index which continuously changes from a value substantially equal to a center refractive index of the rod lens to a value substantially equal to the refractive index of the optical fiber along a film thickness direction of the anti-reflection film. The optical fiber collimator further includes a refractive index matching medium which has a refractive index substantially equal to that of the optical fiber and which couples the anti-reflection film to an end face of the optical fiber.
According to another aspect of the present invention, there is provided a method for manufacturing an optical fiber collimator including a gradient index rod lens and an optical fiber optically connected to the rod lens. The method comprises a step of forming an anti-reflection film on one end face of the rod lens. The anti-reflection film has a refractive index which continuously changes from a value substantially equal to a center refractive index of the rod lens to a value substantially equal to the refractive index of the optical fiber along a film thickness direction of the anti-reflection film. The method further comprises a step of using a refractive index matching medium having a refractive index substantially equal to that of the optical fiber to bond the anti-reflection film to the end face of the optical fiber.
According to further aspect of the present invention, there is provided an optical fiber collimator array comprising a plurality of optical fiber collimators. Each of the plurality of optical fiber collimators includes a gradient index rod lens and an optical fiber optically connected to the rod lens. Each optical fiber collimator includes an anti-reflection film formed on one end face of the rod lens. The anti-reflection film has a refractive index which continuously changes from a value substantially equal to a center refractive index of the rod lens to a value substantially equal to the refractive index of the optical fiber along a film thickness direction of the anti-reflection film. The optical fiber collimator further includes a refractive index matching medium which has a refractive index substantially equal to that of the optical fiber and which bonds the anti-reflection film to an end face of the optical fiber.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.